(1) Field of the Invention
The present invention relates to a fabrication method for a semiconductor apparatus, in particular, to a method for forming gold lines.
(2) Description of the Prior Art
In conventional integrated circuits composed of GaAs compound semiconductor devices or the like, gold is preferably used because it has a low resistance and a migration resistance characteristic. When gold lines are formed, since the deposition rate of the sputtering method or the like is low, in order to form thick films on the level of 1 .mu.m, a plating method has been widely used.
Conventionally, gold lines are formed step by step in the following manner (referred to as a first prior art reference).
As shown in FIG. 4(a), a base substrate of which an insulation film 2 with connection holes such as contact holes (not shown) are formed on the front surface of a base substrate such as a substrate 1 is prepared. As shown in FIG. 4(b), a titanium film 3 with a thickness of for example 100 nm and a platinum film 4 with a thickness of for example 100 nm are deposited by evaporating process or sputtering process. As shown in FIG. 4(c), a photoresist film 6 with openings 5 in a predetermined pattern is formed. With a current supplying layer of the platinum film 4 and the titanium film 3, a gold plate film 7 with a thickness of for example 1 .mu.m is formed. As shown in FIG. 4(d), the photoresist film 6 is removed. The two-layered films that are not coated with the gold plate film 7 are removed. Thereby, gold lines 8 are formed.
When such integrated circuits are used in a high frequency region, the most important point to be considered as lines is a current loss due to a high frequency current. In other words, when an electromagnetic wave propagates through lines, a current flows near the surface of each line. In other words, skin effect takes place. As the frequency increases, the thickness of the surface in which the current flow decreases. However, the resistance per unit length of each line is inversely proportional to the sectional area of the portion in which the current flows. Thus, as the frequency increases, the resistance increases. Thereby, the current loss increases. Thus, when lines with a rectangular section are used even if the resistance of the material thereof is low, the current loss becomes large. To solve this problem, the thickness of the plate is increased so as to increases the surface area of the lines. In addition, the section of each line is formed in a U shape (hereinafter, referred to as U-shaped gold line). This method was proposed in "1992 Autumn Conference Report Separate Issue 2", page 420, The Institute of Electronics, Information, and Communication Engineers of Japan.
Next, the method for forming U-shaped gold lines will be described as a second prior art reference. As shown in FIG. 5(a), a photoresist film 9 with a thickness of for example 10 .mu.m is formed at a predetermined portion of an insulation film 2 (having connection holes such as contact holes (not shown)) on a semiconductor substrate such as a GaAs substrate 1 by the known photolithography technology. Next, as shown in FIG. 5(b), by the sputtering method, a metal film as a current supplying layer 10 is formed on the entire front surface. As shown in FIG. 5(c), a gold plate film 11 with a thickness of 1 .mu.m is formed. Thereafter, as shown in FIG. 5(d), a photoresist film 12 as a mask material is formed at a portion where lines are formed. The current supplying layer of the gold plate film is removed from non-line portions by the ion-milling process. Finally, the photoresist films 12 and 9 are peeled off. Thereby, as shown in FIG. 5(e), U-shaped gold lines 13 are formed.
According to the first and second prior art references, since metal films are removed by the ion-milling method, the metal particles which were removed from the ground substrate by the ion-milling method tend to re-adhere to the inside of the ion-milling unit. Thus, a first problem is that it is difficult to clean the inside of the ion-milling unit. Another problem is that, conductive particles tend to cause short-circuits between lines.
In the ion-milling step, the metal particles of ion-milled platinum, gold and the like tend to re-adhere to the lower portion and side surface of the lines. Thus, the lines are problem.
The first and second problems become serious as the integrating degree of integrated circuits rises.
The second problem can be solved by forming the openings of the photoresist film 6 in such a manner that the bottom area of each opening is larger than the top area thereof. Namely, each section of the gold plating film 7 is formed in a trapezoidal shape. This method is described in Japanese Patent Laid-Open Publication No. 63-292630. However, in this method, the sectional area of each gold line becomes small and thereby the resistance thereof increases. Thus, wider lines should be designed. Consequently, this method does not satisfy the requirement of fine line structure.